1. Field of the Invention
The present invention generally relates to illuminators used in conjunction with inspection systems, such as semiconductor wafer inspection systems and photomask inspection systems, and more particularly to a fiber amplifier based light source for use with such inspection systems.
2. Description of the Related Art
FIG. 1(A) is a diagram depicting a simplified UV-DUV laser inspection system 40 utilized in the semiconductor industry for inspecting a target sample (e.g., a wafer or photomask/reticle) 41. Inspection system 40 includes an illumination source 50 that generates laser light L50 typically in the UV-DUV range (e.g., a vacuum wavelength less than 350 nm), an optical system 42 including one or more objective lenses 43 that focus the laser light onto sample 41, a pass-through detector assembly 45-1 including a sensor array 46-1 positioned to receive any laser light that passes through sample 41 (e.g., for purposes of inspecting a photomask/reticle), and a reflected light detector assembly 45-2 including a sensor array 45-2 positioned to detect any laser light that is reflected from sample 41 (e.g., for purposes of inspecting wafer surface features). Note that where a wavelength is stated herein without qualification, it is assumed to refer to the wavelength in vacuum. The vacuum wavelength λ=c/ν, where c is the velocity of light in vacuum and ν is the frequency in cycles per unit time. The angular frequency, ω, is equal to 2 πν and is in units of radians per unit time. A controller (computing system) 47 controls the operation of the various subsystems according to a software operating program, and processes image data received from detector assemblies 45-1 and 45-2 using techniques known in the art.
It is understood that, in general, shorter wavelength laser light produces higher resolution images, which in a laser inspection system provides better information regarding features and defects on the imaged samples. To meet the increasing demand for laser inspection systems having ever higher resolution, the current trend in the semiconductor industry is toward the development of short wavelength UV-DUV laser inspection systems (i.e., systems utilizing laser light below 250 nm). For example, the assignee of the present application is currently working to develop low frequency UV-DUV laser inspections systems operating with 213 nm, 206 nm or 193 nm laser light.
A significant obstacle to the development of short wavelength UV-DUV laser inspection systems is to provide an optical system that can effectively image the UV laser light. The only two practical materials available for generating the various lenses and elements for the optical system of a UV-DUV laser inspection system (e.g., optical system 42 in FIG. 1(A)) are fused silica and calcium fluoride, with fused silica being preferred because calcium fluoride is much more expensive to obtain, polish and mount. Optical systems manufactured from all fused silica for use in systems using UV-DUV laser light can only handle a limited bandwidth before the performance degrades beyond acceptable limits. Specifically, the larger the numerical aperture (NA) and field size, and the shorter the wavelength, the smaller the acceptable bandwidth can be. For example, an all-refractive objective 266 nm with 0.8 NA and 1.0 mm field of view may only achieve a bandwidth of 5 pm. One approach to deal with a larger bandwidth this is to reduce the glass path by using aspheric surfaces because a single aspheric surface may eliminate several equivalent spherical lenses. However the increased cost and complexity associated with the use of aspheric surface may not be desirable, and this approach only helps a small amount in most laser systems.
To minimize the cost and complexity required to generate optical system 42 for short wavelength UV-DUV laser inspection system 100, illumination source 50 must be able to generate laser light L50 in which substantially all of the light energy is within a narrow bandwidth. It is typical to specify the bandwidth of a laser light source using a full width half maximum (FWHM) value, which specifies the light's bandwidth range at one-half of the light's peak power. However, in UV-DUV laser inspection systems, the bandwidth range at which 95% of the energy is contained (i.e., the light's “E95” bandwidth value) is the more important value. A typical illumination source 41 generates laser light L50 having a relatively narrow FWHM bandwidth value, but having an E95 value that is ten or more times broader than it's FWHM. It is therefore important in laser imaging system 40 to utilize an illumination source 50 that generates narrow band UV laser light L50 that is both short wavelength UV (e.g., laser light having a nominal wavelength value below 250 nm) and has a narrow E95 bandwidth (i.e., within ±1%, and preferably within ±0.1%, of the nominal or “central” UV frequency).
Narrow band UV light is typically created by generating fundamental light having a longer wavelength (typically longer than 1 micron), and then converting the fundamental light using crystals that perform nonlinear frequency conversion and frequency mixing to generate UV light having a desired (shorter) wavelength. Because of limitations on the frequency conversion/mixing process, the fundamental light must have a specific higher frequency in order to generate UV light at a specified shorter wavelength. It is also possible to perform the frequency conversion/mixing process using other nonlinear processes (e.g., Raman, parametric generation, and four wave mixing (FWM)), but these techniques can also lead to increased bandwidths and not be suitable for narrow bandwidth optics. Many stages of frequency conversion/mixing are sometimes needed to generate shorter wavelength light having a specified frequency, and power is lost from the light during each frequency conversation stage. Therefore, in order to generate UV laser light at an acceptable power, it is necessary to generate the fundamental light at significantly higher peak power than is needed at the optical system.
There are two types of fundamental light sources used in the generation of narrow band UV light: solid state lasers and fiber lasers. Solid-state lasers can produce laser light having very narrow bandwidths and high peak power, which allows for the use of less complex (and therefore lower cost) optical systems, but the wavelength choices for solid state lasers are very limited and not suitable for some laser inspection systems, and it can be very challenging to obtain reliable high power light from a solid state laser. Fiber lasers include an active gain medium formed by an optical fiber doped with rare-earth elements such as Erbium, Ytterbium, Neodymium, Dysprosium, Holmium, Praseodymium, and Thulium. Fiber lasers are an attractive choice for generating fundamental light in laser inspection systems because they generate laser light having high peak power, and the frequency of the laser light can be “tuned” to a specified frequency by altering the amounts of doping materials in the fiber(s). However, as described below, the primary drawback of using fiber lasers to generate high peak power pulsed fundamental light is Self Phase Modulation (SPM). In general, SPM is a nonlinear optical effect of light-matter interaction, where ultrashort light pulses travelling in the fiber medium induce a varying refractive index of the medium due to the optical Kerr effect. The variation in refractive index produces a phase shift in the light pulse, leading to a change of the pulse's frequency spectrum. The nonlinear SPM effect can dramatically increase the spectral bandwidth of a fiber laser well beyond the optical requirements of a laser inspection system.
FIG. 1(B) is a diagram showing a conventional fiber-based illumination source 50, which is utilized to generate UV laser light L50 in inspection system 40 (shown in FIG. 1(A)). Fiber-based illumination source 50 generally includes a fundamental light source 51 for generating fundamental light F51 at a specified fundamental frequency ωF, and a frequency conversion module 55 that performs the frequency conversion/mixing process mentioned above in order to generate UV laser light L50 at a specified UV frequency ωUV that is passed to optical system 42 (see FIG. 1(A)). Fundamental light source 51 includes a seed laser 52 that generates seed light S52 having the desired fundamental light frequency ωF at an initial power P0, a pump laser 53 that generates pump seed light PS at a suitable seed frequency ωS, and one or more fiber amplifiers 54 that utilize the pump seed light PS to amplify seed light S52 in a manner understood in the art, whereby fundamental light F51 is produced having the desired fundamental frequency ωF and an amplified power PA that is substantially higher than initial power P0. Fundamental light F51 is then converted/mixed by frequency conversion module 55 to generate UV laser light L50 having the desired UV frequency ωUV, but at an output power POUT that is substantially lower than the amplified power PA of fundamental light F51 (i.e., due to energy losses during the conversion/mixing process).
As mentioned above, fundamental light F51 has a bandwidth ΔωF that is determined in part by the SPM characteristics of fiber amplifier(s) 54, as is well known in the art. SPM gives rise to a phase shift during the amplification process that is intensity dependent given by:
                    ϕ        NL            ⁡              (                  L          ,          T                )              =                                                  U            ⁡                          (                              O                ,                T                            )                                                2            ⁢              (                              L            eff                                L            NL                          )            ⁢                          ⁢      where                  L      eff        =                            1          -                      exp            ⁡                          (                                                -                  α                                ⁢                                                                  ⁢                L                            )                                      α            ⁢                          ⁢      and                  L      NL        =          1              γ        ⁢                                  ⁢                  P          0                    In the above equations, φNL is the intensity dependent phase shift, L is the fiber length, T is time, U is the energy distribution, α is the fiber loss, Leff is the effective length of the fiber considering fiber loss, LNL is the fiber length at which significant SPM occurs, P0 is the peak power of the pulse, and γ is the nonlinear coefficient. Because one of the seed light S52 or the pump seed light PS are pulsed, the intensity φNL changes in time, and this produces a phase that changes in time as well. When the phase of light changes in time, this creates changes in the wavelength spectrum. The spectral shift δω(T) relative to the central frequency value is given by
      δω    ⁡          (      T      )        =            -                        ∂                      ϕ            NL                                    ∂          T                      =                  -                  (                                    L              eff                                      L              NL                                )                    ⁢              ∂                  ∂          T                    ⁢                                              U            ⁡                          (                              0                ,                T                            )                                                2            
Spectral shift δω(T) is also known as a chirp, or change in the instantaneous frequency across the pulse.
FIGS. 2(A) and 2(B) are optical spectrum diagrams showing examples of a seed pulse S52 and a fundamental light pulse F51 generated by conventional fundamental light source 51 (shown in FIG. 1(B)), and illustrate the output bandwidth produced by the intensity dependent phase shift generated in conventional fiber-based illumination source 50 (FIG. 1(B)). FIG. 2(A) shows that fundamental seed light S52 is an initial transform limited Gaussian pulse has a peak power of approximately 6×106 W centered around a fundamental frequency ωF (e.g., the frequency corresponding to a wavelength of 1030 nm) and having an FWHM value of 11 GHz and an E95 energy bandwidth of 23 GHz. FIG. 2(B) shows fundamental light pulse F52, which is produced in amplifier 54 (FIG. 1(B)) using seed light pulse S52 and pump seed light PS. FIG. 2(B) shows that fundamental light pulse F52 remains generally centered around fundamental frequency ωF while being amplified to a peak power PA of 10 kW. However, due to the SPM characteristics of fiber amplifier(s) 54, the FWHM value of fundamental light pulse F52 increases to 222 GHz, and exhibits an E95 energy bandwidth of 286 GHz. This roughly tenfold increase in E95 energy exhibited by fiber-based fundamental light sources is impractical for generating the type of narrow band UV light needed in modern laser inspection systems.
What is needed is a fundamental light source that combines the high peak power and frequency-tuning capability of fiber-based lasers with the simplified (low cost) optical systems typically associated with solid-state lasers.